In motor vehicles in which natural gas (main component: methane) is used as a fuel, the techniques employed for storing natural gas include a method of filling a vessel with compressed natural gas and a method of filling a vessel with liquefied natural gas.
However, the former technique has problems that the equipment cost is high because vessels are required to withstand a high pressure (about 20 MPa) and that poor suitability of the vessels in mounting on vehicles results in the reduction in the capacity, e.g., that of the trunk. On the other hand, the latter technique has problems that a vacuum double-wall vessel is necessary since liquefied natural gas should be kept at a very low temperature (-162.degree. C.), and that an apparatus for vaporizing the liquefied natural gas is also necessary. As a result, the equipment cost in the latter technique is as high as in the former.